LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Christian Borntraeger <borntraeger@de.ibm.com>
To: peterz@infradead.org
Cc: borntraeger@de.ibm.com, bristot@redhat.com, bsegall@google.com,
	dietmar.eggemann@arm.com, joshdon@google.com,
	juri.lelli@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
	linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org,
	rostedt@goodmis.org, valentin.schneider@arm.com,
	vincent.guittot@linaro.org
Subject: [PATCH 1/1] sched/fair: improve yield_to vs fairness
Date: Wed,  7 Jul 2021 14:34:02 +0200	[thread overview]
Message-ID: <20210707123402.13999-2-borntraeger@de.ibm.com> (raw)
In-Reply-To: <20210707123402.13999-1-borntraeger@de.ibm.com>

After some debugging in situations where a smaller sched_latency_ns and
smaller sched_migration_cost settings helped for KVM host, I was able to
come up with a reduced testcase.
This testcase has 2 vcpus working on a shared memory location and
waiting for mem % 2 == cpu number to then do an add on the shared
memory.
To start simple I pinned all vcpus to one host CPU. Without the
yield_to in KVM the testcase was horribly slow. This is expected as each
vcpu will spin a whole time slice. With the yield_to from KVM things are
much better, but I was still seeing yields being ignored.
In the end pick_next_entity decided to keep the current process running
due to fairness reasons.  On this path we really know that there is no
point in continuing current. So let us make things a bit unfairer to
current.
This makes the reduced testcase noticeable faster. It improved a more
realistic test case (many guests on some host CPUs with overcomitment)
even more.
In the end this is similar to the old compat_sched_yield approach with
an important difference:
Instead of doing it for all yields we now only do it for yield_to
a place where we really know that current it waiting for the target.

What are alternative implementations for this patch
- do the same as the old compat_sched_yield:
  current->vruntime = rightmost->vruntime+1
- provide a new tunable sched_ns_yield_penalty: how much vruntime to add
  (could be per architecture)
- also fiddle with the vruntime of the target
  e.g. subtract from the target what we add to the source

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 kernel/sched/fair.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 23663318fb81..4f661a9ed3ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7337,6 +7337,7 @@ static void yield_task_fair(struct rq *rq)
 static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)
 {
 	struct sched_entity *se = &p->se;
+	struct sched_entity *curr = &rq->curr->se;
 
 	/* throttled hierarchies are not runnable */
 	if (!se->on_rq || throttled_hierarchy(cfs_rq_of(se)))
@@ -7347,6 +7348,16 @@ static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)
 
 	yield_task_fair(rq);
 
+	/*
+	 * This path is special and only called from KVM. In contrast to yield,
+	 * in yield_to we really know that current is spinning and we know
+	 * (s390) or have good heuristics whom are we waiting for. There is
+	 * absolutely no point in continuing the current task, even if this
+	 * means to become unfairer. Let us give the current process some
+	 * "fake" penalty.
+	 */
+	curr->vruntime += sched_slice(cfs_rq_of(curr), curr);
+
 	return true;
 }
 
-- 
2.31.1


  reply	other threads:[~2021-07-07 12:35 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-12 10:14 [PATCH v2 0/9] sched: Clean up SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 1/9] sched/numa: Allow runtime enabling/disabling of NUMA balance without SCHED_DEBUG Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 2/9] sched: Remove sched_schedstats sysctl out from under SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 3/9] sched: Dont make LATENCYTOP select SCHED_DEBUG Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] sched: Don't " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 4/9] sched: Move SCHED_DEBUG sysctl to debugfs Peter Zijlstra
2021-04-15 16:29   ` [PATCH] sched/debug: Rename the sched_debug parameter to sched_debug_verbose Peter Zijlstra
2021-04-19 19:26     ` Josh Don
2021-04-16 15:53   ` [tip: sched/core] sched: Move SCHED_DEBUG sysctl to debugfs tip-bot2 for Peter Zijlstra
2021-04-27 14:59   ` Christian Borntraeger
2021-04-27 15:09     ` Steven Rostedt
2021-04-27 15:17       ` Christian Borntraeger
2021-04-28  8:47       ` Peter Zijlstra
2021-04-28  8:46     ` Peter Zijlstra
2021-04-28  8:54       ` Christian Borntraeger
2021-04-28  8:58         ` Christian Borntraeger
2021-04-28  9:25         ` Peter Zijlstra
2021-04-28  9:31           ` Christian Borntraeger
2021-04-28  9:42       ` Christian Borntraeger
2021-04-28 12:38         ` Peter Zijlstra
2021-04-28 14:49           ` Christian Borntraeger
2021-07-07 12:34           ` [PATCH 0/1] Improve yield (was: sched: Move SCHED_DEBUG sysctl to debugfs) Christian Borntraeger
2021-07-07 12:34             ` Christian Borntraeger [this message]
2021-07-07 18:07               ` [PATCH 1/1] sched/fair: improve yield_to vs fairness kernel test robot
2021-07-23  9:35               ` Mel Gorman
2021-07-23 12:36                 ` Christian Borntraeger
2021-07-23 16:21                   ` Mel Gorman
2021-07-26 18:41                     ` Christian Borntraeger
2021-07-26 19:32                       ` Mel Gorman
2021-07-27  6:59                         ` Christian Borntraeger
2021-07-27 18:57                       ` Benjamin Segall
2021-07-28 16:23                         ` Christian Borntraeger
2021-08-10  8:49                           ` Vincent Guittot
2021-07-27 13:29                     ` Peter Zijlstra
2021-07-27 13:33                 ` Peter Zijlstra
2021-07-27 14:31                   ` Mel Gorman
2021-04-12 10:14 ` [PATCH v2 5/9] sched,preempt: Move preempt_dynamic to debug.c Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 6/9] debugfs: Implement debugfs_create_str() Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 7/9] sched,debug: Convert sysctl sched_domains to debugfs Peter Zijlstra
2021-04-13 14:55   ` Valentin Schneider
2021-04-15  9:06     ` Peter Zijlstra
2021-04-15 12:16       ` Dietmar Eggemann
2021-04-15 12:34       ` Valentin Schneider
2021-04-15 13:02         ` Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 8/9] sched: Move /proc/sched_debug " Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-04-12 10:14 ` [PATCH v2 9/9] sched,fair: Alternative sched_slice() Peter Zijlstra
2021-04-12 10:26   ` Peter Zijlstra
2021-04-16 15:53   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210707123402.13999-2-borntraeger@de.ibm.com \
    --to=borntraeger@de.ibm.com \
    --cc=bristot@redhat.com \
    --cc=bsegall@google.com \
    --cc=dietmar.eggemann@arm.com \
    --cc=joshdon@google.com \
    --cc=juri.lelli@redhat.com \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-s390@vger.kernel.org \
    --cc=linux@rasmusvillemoes.dk \
    --cc=mgorman@suse.de \
    --cc=mingo@kernel.org \
    --cc=peterz@infradead.org \
    --cc=rostedt@goodmis.org \
    --cc=valentin.schneider@arm.com \
    --cc=vincent.guittot@linaro.org \
    --subject='Re: [PATCH 1/1] sched/fair: improve yield_to vs fairness' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).